Brain lesions that develop in neurodegenerative diseases such as Alzheimer's (AD) and Parkinson's are composed of fibrils of 'misfolded'proteins. Consequently much recent research has been focused on molecular chaperones, the cellular machinery responsible for maintaining protein folding or mediating the degradation of irreparably damaged proteins. Of the molecular chaperones, heat shock protein 90 (Hsp90) has emerged as a major hub for regulation of multi-chaperone complexes that allows for cell specific responses to stresses and protein denaturation. Evidence suggests that inhibitors of Hsp90 activate specific chaperone complexes, namely Hsp90, Hsp70, and CHIP. Activation or up-regulation of these chaperone complexes reduces amyloid peptide (Ap) aggregates and fibrillar Tau protein in transgenic mouse models of AD neuropathology. However, most known Hsp90 inhibitors are quite toxic. We have identified two new Hsp90-targeted agents that markedly protect primary neurons in culture against toxicity elicited by A[unreadable] peptides and reduce abnormal Tau, but they do not produce any toxicity on their own. The compounds are protective for neurons at low nanomolar concentrations and appear to cross the blood brain barrier. The overall goal of the proposed research program is to identify, through an integrated, iterative, and rational drug discovery program, development candidates for GLP and GMP first-inhuman enabling studies. Preclinical proof-of-concept for novel Hsp90 inhibitors will be determined by characterizing the in vivo effects of the most promising chemical lead candidates in the triple transgenic (3xTg-AD) mouse model for AD that develops memory deficits and both A[unreadable] plaques and NFT-like Tau aggregates in the brain with increasing age. The specific aims of the program are: (1) To synthesize gram quantities of promising chemical lead candidates, the first being designated as 'KU32'and 'A4, to permit full characterization of the in vivo proof-of-concept and drug safety properties of these agents. (2) To test the in vivo efficacy of promising chemical lead candidates, the first two being KU32 and A4, in reversing or 'treating'the cognitive impairment and neuropathological lesions normally present in the brains of older 3xTg-AD mice. (3) To test the in vivo efficacy of chronic administration of promising chemical lead candidates, the first two being KU32 and A4, in preventing or delaying the appearance of cognitive impairments in the 3xTg-AD mouse model. (4) To identify development candidates that possess improved efficacy, drug safety and 'druggability'properties. Identification of a candidate or candidates with these properties will lay the foundation for submission of an Investigational New Drug Application. There is a strong experimental basis for targeting Hsp90 protein complexes for therapeutic interventions in AD and other neurodegenerative diseases characterized by accumulations of aggregated proteins. Successful completion of this program will advance this concept toward clinical development.